M.Sc. / Donor-acceptor (D-A) systems consist of a donor covalently or non-covalently linked to an acceptor. The simplest D–A system consists of a donor linked to an acceptor and is called a dyad system. Photoactive dyad systems are molecular devices designed to perform through the separation of charge separation states and the conversion of solar energy to chemical energy in analogy to photosynthesis.1 These dyad systems consist of a donor which is usually a chromophore and an acceptor. The design of these systems is guided to mimic photo-induced electron transfer (PET) and charge separation (CS), which are fundamental processes of photosynthesis. In nature, photosynthetic units are often built from dyads consisting of pigments like chlorophyll (donors), non–covalently linked to quinones, (acceptors). The donor harvests light energy and transfers the energy to the nearby pigment molecules until it eventually reaches a special region of the chlorophyll macrocycle called the reaction centre where this light energy is then converted to electrochemical energy. Photoactive dyad systems act as artificial photosynthetic models as they reproduce photo–induced electron transfer and charge separation of natural photosynthesis. In this project, dyad systems were made by covalently linking zinc pheophorbide, a modified chlorophyll derivative to double–walled carbon nanotubes (DWCNTs). Zinc pheophorbide acts as the donor and DWCNTs as the acceptors. Chlorophyll was modified by cutting the phytol chain and inserting zinc as the central metal to yield zinc pheophorbide. This derivative is stable against irradiation, has a good range of acceptor wavelength and is also a good light harvester. DWCNTs are one dimensional nanowires with two concentric tubes. They readily accept electrons because they have an extended π electron system. These electrons are then transported efficiently under ballistic conditions. DWCNTs were synthesised by catalytic chemical vapour deposition (CCVD) of methane over Mg0.99Co0.075Mo0.025O catalyst. In dyad system 1, amidated zinc pheophorbide molecules were covalently attached to oxidised DWCNTs in the presence of N–ethyl–N’–(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) \ and N–hydroxysuccinnimide (NHS) as a catalysts. Dyad system 2 was synthesized by attaching zinc pheophorbide molecules to amidated DWCNTs using the same catalysts.

Poster exhibited at GPSC Student Showcase, February 24th, 2016, University of Arizona. Recipient of the 2016 Katheryne B. Willock Library Research Award. / The nervous system is composed of two types of cells: neurons and glia. In neuronal circuits, neurons communicate through synapses and glia play a crucial modulatory role. To modulate chemical reuptake, glia send processes close to synapses and many glia directly appose or ensheathe a synapse. This structural motif is one of the elements often included in describing a vertebrate tripartite synapse, which includes a bidirectional functional neuron-glia relationship. The exact nature of this neuron-glia communication is not well understood. In the invertebrate fruit fly, we have also found that particular neurons and glia also have a bidirectional functional relationship. This allows us to ask new questions about glial morphology. Throughout multiple images, I identified particular neuronal synapses and surrounding glia. After creating a 3D reconstruction, I measured the distance between a particular neuronal synapse and its closest glial process. Interestingly, the neuronal synapses were not directly apposed or ensheathed by glia, and the distance to the closest glial process varied one-hundred-fold. With variable distance, functional communication is consistently present. These findings provide important insight into invertebrate neuron-glia communication, and offer new avenues to investigate the structural neuron-glia relationships that are required for reciprocal signaling between the two cell classes.

No description available.

No description available.

The three-dimensional (3-D) structure of gp32*I, a major proteolytic fragment of the DNA helix-destabilizing protein from bacteriophage T4, has been determined at 18 A resolution by electron microscopy of negatively stained crystals and computer image analysis. The crystalline areas processed in 3-D have the symmetry of the space group P2₁, with a = 47 Å, b = 63 Å, c = 65 Å, and α = β = γ = 90°. This P2₁ unit cell contains one gp32*I molecule per asymmetric unit. The molecule is roughly V-shaped, containing two large domains linked by a smaller domain occupying the base of the V. The total length of the molecule is about 110 Å with an average diameter of about 25 Å. Systematic analysis of the symmetry in images of untilted crystals determined that the crystal could display several types of projection symmetry, pgg, pg corresponding to P2₁ symmetry with the screw axis along the a axis of the crystal, and pg corresponding to P2₁ symmetry with the screw axis along the b axis. Among images displaying pg symmetry along the b axis, two types of images with noticeably different appearances were obtained. A hypothesis was formed that explained the different types of symmetry as the result of the growth of the gp32*I crystal in the space group P2₁ 2₁ 2₁, in steps of 1/2 of a unit cell along the thin direction of the crystal. Two different types of 1/2 unit cell thick steps were postulated. Computer simulations were used to generate synthetic images of untilted crystals containing either one, two or three steps of each kind. The results of the simulations prove that the space group of the gp32*I crystal is P2₁ 2₁ 2₁. They suggest that careful analysis of the symmetry in images of untilted gp32*I crystals can provide information about the thickness of the crystals. A strategy is presented for determining the structure of the gp32*I crystal at higher resolution by electron microscopy of frozen, hydrated crystals. This strategy includes the use of symmetry analysis as a tool for determining the thickness of the crystals so that data from crystals of the same thickness can be combined in 3-D. A similar approach may prove useful in the 3-D electron microscopic analysis of other thin, multi-layered crystals.

Detectors for Electron Microscopy have traditionally used a scintillator to generate photons from fast electrons, which are then detected by a sensor. However, in recent years direct detection has become an area of interest due to the potential improvements to detector performance. In this thesis various aspects of direct detection are presented. I will begin with simulations of direct detectors based on Joy’s model of straight trajectories between Rutherford scattering events, where signal is generated by inelastic scattering events. The effects of microscope operating voltage, detector thickness, a surface electrically dead layer and diode depth on detector performance are presented. A prototype detector was developed using the DUOS sensor, two thicknesses of the sensor were produced a 50μm thick detector and a 20μm thick detector. EBSD results are presented which show how the use of a reactive ion etch to reduce the dead layer thickness of a mechanically thinned sensor improve the detection efficiency of a sensor allowing EBSD work to be carried out at operating voltages as low as 5keV. The MTF and DQE of both thicknesses of DUOS sensor are measured at 80kV and 200kV, which show that there is little difference between the two thicknesses at 80kV, but at 200kV the thinner detector shows an improved MTF. The results are then and compared with the equivalent simulated detectors. I show how the high frame rate of a detector and rigid and non-rigid registration can be used to improve image quality, resolving the {331} lattice spacing which is not visible with a simple summation of frames. Detectors using gallium nitride rather than silicon as the base semiconductor are simulated. The MTF at the Nyquist frequency for a GaN detector is double that of a Si detector at an operating voltages of 80kV due to the smaller interaction volume of an electron in GaN. However, at higher voltages the improvement is much smaller as most electrons pass through the detector.

No description available.

Abstract Diamond is nowadays regarded as a potential semiconductor material of the future, due to its extreme and unique properties. Some of these properties, in- clude its high hardness, highest breakdown ¯eld, high Debye temperature, high thermal conductivity, high hole and electron mobilities, large bandgap and op- tical transparency, among others. These properties make diamond suitable for high-temperature, high-speed and high-power electronic applicatons, as well as in other applications. However, defects associated with ion implantation have been shown to make it rather di±cult to obtain n-type doping in diamond. As such, an understanding of the nature of defects produced during ion implanta- tion of diamond remains a subject of great importance, if not essential, for the optimization of high-temperature, high-power electronic applications in partic- ular. In this respect, this study investigates the nature of the radiation damage generated within the collision cascades of multi-implantations of carbon ions in high-pressure, high-temperature single-crystal synthetic type Ib diamond, spread over a range of energies (50-150keV) and doses. This is achieved by means of the cold-implantation-rapid-annealing (CIRA) routine, and the anal- ysis of damage caused was done by using cross sectional transmission electron microscopy techniques. More precisely, the modes used to achieve this are the bright ¯eld transmission electron microscopy (BFTEM) coupled with selected area di®raction or SAD. At low dose implantation or at sub-critical implantation doses (2.5x1015 ions/cm2), it was found that the ion-damaged diamond layer consists of some threading dislocations, not homogeneously distributed which propagate from the surface into the ion-damaged diamond. In contrast to the sub-critical implantation doses , it was found that at very high implantation doses (7.0x1015 ions/cm2), i.e., above the critical dose (where diamond transforms to graphite upon annealing), the damaged diamond layer had some unconventional defect features close to the implanted surface.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, June, 2013 / Diamond with its outstanding and unique physical properties offers the opportunity to be used as semiconductor material in future device technologies. Promising ap- plications are, among others, high speed and high-power electronic devices working under extreme conditions, such as high temperature and harsh chemical environments. With respect to electronic applications, a controlled doping of the material is neces- sary which is preferably done by ion implantation. The ion implantation technique allows incorporation of foreign atoms at de¯ned depths and with controlled spatial distribution which is not achievable with other methods. However, the ion implanta- tion process is always connected with the formation of defects which compensate and trap charge carriers thus degrading the electrical behaviour. It is therefore essential to understand the nature of defects produced under various implantation conditions. In this respect, this study involves the investigation of the nature of the radiation damage produced during the multi-implantation of carbon ions in synthetic high- pressure, high-temperature (HPHT) type Ib diamond spread over a range of energies from 50 to 150 keV and °uences, using the cold-implantation-rapid-annealing (CIRA) routine. Single energy implantation of carbon ions in synthetic HPHT (type Ib), at room temperature, was also performed. Both ion milling and FIB (Focused Ion Beam) milling were used to prepare thin specimen for transmission electron micro- scope (TEM) analysis. The unimplanted, implanted and annealed samples were characterized using trans- mission electron microscopy based techniques and Raman spectroscopy. ii iii In unimplanted type Ia natural diamond, a high density of platelets, exhibiting the typical contrast of both edge-on and inclined platelets on f100g planes was found. As-implanted HPHT type Ib diamond, implanted with single energy of 150 keV car- bon ions and °uence of 7£1015 ions cm¡2 revealed an amorphous diamond layer of about 80 nm in thickness while, for low °uence implantations, the damaged diamond retained its crystallinity after annealing at 1600 K. In addition, damaged diamond transformed into disordered carbon comprising regions with bent (002) graphitic fringes and regions of amorphous carbon when high °uence, i.e., one above the amor- phization/graphitisation threshold were used followed by rapid thermal annealing at 1600 K. Furthermore, the interface between the implanted and annealed layer and the diamond substrate at the end of the range, showed diamond crystallites, inter- spersed between regions of amorphous carbon and partially graphitized carbon. This indicates that solid phase epitaxial recrystallization regrowth in diamond does not occur.

by Chan, Ray. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves [93-96]). / ACKNOWLEDGMENT / ABSTRACT / FIGURE CAPTIONS / TABLE CAPTIONS / INTRODUCTION / Chapter I. --- PRINCIPLES OF SEM / Chapter 1.1 --- STRUCTURE OF SEM --- p.1-1 / Chapter 1.2 --- IMAGE FORMATION --- p.1-6 / Chapter 1.2.1 --- Electron Beam-Specimen Interaction --- p.1-6 / Chapter 1.2.1.1 --- Electron scattering in solid specimen --- p.1-7 / Chapter 1.2.1.2 --- Electron range and spatial distribution --- p.1-7 / Chapter 1.2.1.3 --- Back scattered electrons(BE) --- p.1-8 / Chapter 1.2.1.4 --- Secondary electron(SE) --- p.1-9 / Chapter 1.2.1.5 --- Other signal types --- p.1-13 / Chapter 1.2.2 --- Types of Image Contrast --- p.1-14 / Chapter 1.3 --- DISTORTION AND NOISE --- p.1-17 / Chapter 1.3.1 --- Lens Aberration --- p.1-17 / Chapter 1.3.1.1 --- Spherical aberration --- p.1-17 / Chapter 1.3.1.2 --- Chromatic aberration --- p.1-18 / Chapter 1.3.1.3 --- Diffraction effect --- p.1-19 / Chapter 1.3.1.4 --- Axial astigmatism --- p.1-20 / Chapter 1.3.1.5 --- Spatial resolution calculation --- p.1-21 / Chapter 1.3.2 --- Image Defects --- p.1-22 / Chapter 1.3.2.1 --- Projection distortion --- p.1-22 / Chapter 1.3.2.2 --- Specimen tilting --- p.1-22 / Chapter 1.3.2.3 --- Moire effects --- p.1-22 / Chapter 1.3.3 --- Noise in SEM --- p.1-23 / Chapter II. --- SEM FOR IC TESTING / Chapter 2.1 --- QUANTITATIVE ANALYSIS OF VOLTAGE MEASUREMENT --- p.2-1 / Chapter 2.1.1 --- Energy Analysis of SEs --- p.2-1 / Chapter 2.1.2 --- Suppression of Local Fields --- p.2-2 / Chapter 2.1.3 --- "Elimination of Topography, Material Contrast and Work Function Variation" --- p.2-2 / Chapter 2.2 --- VOLTAGE RESOLUTION --- p.2-3 / Chapter 2.3 --- TIME RESOLUTION --- p.2-4 / Chapter 2.3.1 --- SE Generation Time --- p.2-4 / Chapter 2.3.2 --- SE Flight Time --- p.2-4 / Chapter 2.3.3 --- Required Voltage Resolution --- p.2-5 / Chapter 2.3.4 --- Electron Beam Pulse Width --- p.2-5 / Chapter 2.4 --- SPATIAL RESOLUTION --- p.2-5 / Chapter 2.5 --- CAPACITIVE COUPLING VOLTAGE CONTRAST --- p.2-6 / Chapter III. --- SEM TESTING TECHNIQUES / Chapter 3.1 --- CONVENTIONAL TESTING METHODS SYNOPSIS --- p.3-1 / Chapter 3.2 --- TESTING TECHNIQUES FOR SEM --- p.3-2 / Chapter 3.2.1 --- Static Mode --- p.3-2 / Chapter 3.2.2 --- Frequency Matching Mode --- p.3-3 / Chapter 3.2.2.1 --- Voltage coding --- p.3-4 / Chapter 3.2.2.2 --- Stroboscopy --- p.3-4 / Chapter 3.2.2.3 --- Waveform measurement --- p.3-6 / Chapter 3.2.2.4 --- Logic state mapping --- p.3-6 / Chapter 3.2.2.5 --- Frequency tracing --- p.3-7 / Chapter 3.2.2.6 --- Frequency mapping --- p.3-8 / Chapter 3.2.2.7 --- Logic state tracing --- p.3-9 / Chapter IV. --- CONVERT AMRAY 1830 INTO E-BEAM TESTER / Chapter 4.1 --- DESIGN CONSIDERATION --- p.4-1 / Chapter 4.1.1 --- Application Consideration --- p.4-1 / Chapter 4.1.2 --- Limitation of the AMRAY 1830 --- p.4-1 / Chapter 4.1.2.1 --- Detection system --- p.4-2 / Chapter 4.1.2.2 --- Scanning driver --- p.4-3 / Chapter 4.1.2.3 --- Beam blanker --- p.4-4 / Chapter 4.1.2.4 --- NibbleNet interface --- p.4-5 / Chapter 4.2 --- HARDWARE ARCHITECTURE --- p.4-6 / Chapter 4.2.1 --- SEM Circuit Varied --- p.4-7 / Chapter 4.2.1.1 --- Adding scanning relays --- p.4-7 / Chapter 4.2.1.2 --- Voltage clippers --- p.4-7 / Chapter 4.2.1.3 --- External scan interface in SEM --- p.4-9 / Chapter 4.2.2 --- PC Interface --- p.4-9 / Chapter 4.2.3 --- Driver Box --- p.4-10 / Chapter 4.2.3.1 --- Data preprocess unit --- p.4-10 / Chapter 4.2.3.2 --- Scanning preprocess unit --- p.4-12 / Chapter 4.2.3.3 --- Control unit --- p.4-12 / Chapter 4.2.4 --- Scanning Generation and Data Acquisition --- p.4-13 / Chapter 4.3 --- SOFTWARE DEVELOPED --- p.4-13 / Chapter 4.3.1 --- Function Library --- p.4-13 / Chapter 4.3.2 --- Integrated Environment --- p.4-14 / Chapter V. --- SYSTEM PERFORMANCE / Chapter 5.1 --- CHARACTERISTICS OF THE SCANNING DRIVERS --- p.5-1 / Chapter 5.1.1 --- Driver Output Changes with Acceleration Voltage --- p.5-2 / Chapter 5.1.2 --- Driver Output Changes with magnification --- p.5-3 / Chapter 5.1.3 --- Frequency Response --- p.5-4 / Chapter 5.1.4 --- Image Distortion --- p.5-7 / Chapter 5.2 --- INTEGRATED STUDY ENVIRONMENT --- p.5-9 / Chapter 5.2.1 --- Setting Status --- p.5-9 / Chapter 5.2.2 --- Image Scanning & Image Saving --- p.5-12 / Chapter 5.2.3 --- Static Probing --- p.5-15 / Chapter 5.2.4 --- Point Probing --- p.5-19 / Chapter 5.2.5 --- Frequency Matching --- p.5-20 / Chapter V. --- SUMMARY --- p.6-1 / REFERENCE / APPENDIX: / Chapter A. --- PROGRAM LISTING / Chapter B. --- CIRCUIT SCHEMATICS